Superheterodyne receiving circuit



Nov. 29, 1955 B. e. DAMMERS EIAL 2,725,468

7 SUPERHETERODYNE RECEIVING CIRCUIT Filed June 28, 1952 $2 .2lA/VE/VTORS Bernhordus Gerhurdus Dommers Abraham Gerfrudas Wilhelmusifjens AGE/VT United States Patent Ofiice SUPERHETERODYNE RECEIVINGCIRCUIT Application June 28, 1952, Serial No. 296,215 Claims priority,application Netherlands July 2, 1951 4 Claims. (Cl. 250-20) Theinvention relates to superheterodyne receiving circuits of the kind inwhich use is made of a mixing tube acting as an oscillator by reason ofan inductive coupling between an inductor of a tuned local-oscillatorcircuit in the circuit of a current-conveying electrode of the mixer anda feedback coil included in the circuit of a controlgrid of the tube,the intermediate-frequency oscillations being taken from an impedanceincluded in the anode circuit of the mixer. If the incoming signaloscillations and the local oscillations produced by the mixer itselfoccur on the same control-grid the mixer acts as an additive mixer.

Additive mixing circuits in general and self-oscillating additive mixersin particular have a limitation in that it is difiicult to prevent unduecoupling between the circuit tuned to the local oscillations and theaerial circuit, so that there is always a certain radiation of the localoscillations from the aerial. If a screen-grid tube is employed as amixer and the screen-grid has a certain voltage of the frequency ofthe'local-oscillations, as is the case with several known arrangements,there is a further limitation in that the conversion conductance iscompara tively low. This is due to the fact that the voltages oflocal-oscillator frequency occurring at the anode and at the screen-gridare in phase opposition to the local-oscillator voltage at the controlgrid, so that dc-mixing occurs.

The invention has for its object to mitigate these disadvantages.

According to the invention a superheterodyne receiver in which use ismade of a mixing tube acting as an oscillator by reason of an inductivecoupling between an inductor of a tuned local-oscillator circuit in thecircuit of a current-conveying electrode of the mixer, and a feedbackcoil included in the circuit of a control-grid of the tube, theintermediate-frequency oscillations being taken from an impedanceincluded in the anode circuit of the tube, is characterized in that theincoming signal oscillations are supplied to a tapping point, forexample the centre, of the feedback coil and the said current-conveyingelectrode is connected to a tapping of the inductor of the circuit tunedto the local-oscillator frequency.

If the mixer employed is a tube comprising a screengrid, the screen-gridis preferably connected, for highfrequency currents, to thesecond-mentioned tapping. A very advantageous circuit-arrangement isthat in which both the screen-grid and the anode operate as oscillatorelectrodes and are connected to the said tapping.

Since with the use of the invention in the manner just stated a loweroscillator voltage prevails at the screengrid than is the case with aconnection of the screengrid to that end of the circuit tuned to thelocal-oscillator frequency which is not earthed, de-mixing is reduced.This is also the case if the anode is connected to a tapping of the saidcircuit. Moreover, a favourable internal mixer resistance is obtained atthe same time.

With an additive mixing arrangement it has been suggested to reduce theradiation of oscillations of localoscillator frequency from the aerialby connecting the aerial circuit to the centre of the feedback coil andby connecting that end of the feedback coil which is not connected tothe control-grid of the mixer to a point of constant potential through acapacitor. Thus a bridge circuit is obtained, comprising the two partsof the feedback coil, the input capacity and the said capacitorconnected between the coil and the point of constant potential. If thisbridge circuit is balanced, no voltage of oscillator frequency shouldoccur at the centre of the feedback coil.

It has been found, however, that the use of this known arrangement doesnot always lead to reduction of the oscillator radiation below thepermissible value. This is due to the fact that the circuit coupled withthe feedback coil isnct symmetrical and that, consequently, theequilibrium of the bridge is disturbed by the parasitic capacitiesprevailing between the feedback coil and the coilof the said circuit.

When using the invention, it is found that when incorporating such abridge arrangement a much greater reduction of the oscillator radiationmay be obtained. This is due to the fact that the symmetry of theoscillator circuit with respect to the feedback coil is improved, Whilstmoreover, the oscillator voltage at the coil of the circuit is lower.

.With oscillator circuit-arrangements in which the anode circuit of theanode of a receiving tube includes a circuit tuned to the frequency tobe produced and in which the grid circuit comprises a feedback coil, itis known to connect the anode to a tapping point of the inductor of thetuned circuit. This measure has for its object to reduce the effect ofthe prevailing parasitic capacities on the tuning of the circuit.However, with the reception of oscillations in the medium-wave rangethis effect may be completely neglected with mixingcircuit-arrangements. With the circuit-arrangement according to theinvention the object, of an apparently analogous measurev is thereforequite. different.

In order that the invention may be more clearly understood and readilycarried into effect, it will now be described more fully with referenceto the accompanying drawing, wherein:

Fig. 1 shows a first embodiment, and

Fig. 2 shows a slightly modified embodiment.

Referring to Fig. 1, 1 designates an aerial circuit which is inductivelycoupled with an input circuit 2, tunable to the frequency to bereceived. The circuit 2 is connected to earth at one end and isconnected at the other end through the parallel combination of aresistor 3 and a capacitor 4 to the centre of a coil 5. One end of thecoil 5 is directly connected to the first control-grid 9 of a mixer 6and the other end is connected to earth through the parallel combinationof a resistor 13 and a capacitor 14. The elements 3 and 4 operate as agrid leak and a grid capacitor respectively for the tube 6 which acts asan additive mixer.

The coil 5 is inductively coupled with the coil 17 of a circuit 17, 18,19 tuned to the local-oscillator frequency and acts as a feedback coil.In this circuit capacitor 18 is a tuning capacitor gauged to thecapacitor of circuit 2 whilst the capacitor 19 which is fixed orsemi-variable, is a padding element to obtain a substantially constantfrequency difierence between the oscillator circuit and the inputcircuit.

The two halves of the coil 5 constitute a bridge-circult in conjunctionwith the parallel combination of the resistor 13 and the capacitor 14and with the input impedance of the tube 6, to which the said parallelcombination is equivalent. If the bridge is balanced, no oscillations ofthe local-oscillator frequency should occur across the circuit 2 andthus the aerial circuit.

Patented Nov. 29, 1955 However, it isfound that with known bridgearrangements the freedom from radiation is not generally suffi cient,which is due to thefact that the oscil1ator c irchit is "cemented (inthe one hand to the anode or a screen-grid f the tube and "an the otherhand to earth, :so that there cannot be true symmetry, The barasit'ic"eafiacny between the parts of the feedback coil 5 and the'o's'cillatorcoil disturhs the bridgeequilibfiurr'i. V

'A further disadvantage of certain known additive-mix ing arrangementsis that fde-m'ixing occurs owing to the voltages of further tubeelectrodes being 'in "phase opposition to thegrid voltage. c i yAccording to the invention, in order to twine, said disadvantages, thatpositive electrode of the "tube 6 whose circuit includes the oscillatorcircuit is not cone'ctegi to the end of the inductor 17 "of thiscircuit, but "to a tappin 24 thereof, for ;e)amp1'e, thecentr'e. 'Ihus,the voltage of oscillator frequency at the said positive electrode ofthe tube is lower, sfothat de-rnixing is reduced, whilst at the sametime the symmetry of the inductor 17 with respect to the feedback coil 5is improved. 4

In the embodiment shown in Fig, 1 the anode 8 of the rnixer 6 iscor'lriec'ted to a circuit 15, tuned to intermediate frequency andforming with a second tuned circuit 16 an intermediate-frequencybandpass filter,,a s well as to the 'said tapping 24 of the inductoroithe oscillator circuit and at the same time interconnectedscreen-grids 1Q and 12 Within tube 6 are connected through a capacitor20 to the said tapping. The son grids are, "moreover, connected througha supp y 'res stor l l to the positive terminal er the supply source.The aridde is fed th'rough 'the lower part of the coil 17 and throughresistor 23 from the "sam source, Capacitor, 22pmvides a high-frequencyearth connection for the lower end of 17. In this arrangement the twoscreen-grids operate, together with the anode, as oscillator electrodesfor the local oscillation, which has the additional advantage thatautomatic gain control may be obtained {in a simple manner by means of acontrol grid 11 placed between the two screen-grids. In the absence of asignal the control-grid has, at the most, asugnufy negative voltage withrespect to the cathode 7. The controlvoltage may be fderived, in knownmanner, frbrn the in'terr'nediate-freqiiency signal so that it varieswith the mean carrier-amplitude thereof, The c ol-vol tage exertssubstantially no ihflue'n'ce on thef' a ration of the tiihe, "since thegrid 11 "opr es 'as adiswanting grid an the total cathode currentremains enhstantially constant duriifg control.

Fig. 2 shoi'rv's a second embodiment, gu'i'shed from that shown in Fig 1i'nthat the set grids and am nes-Linda are connected to th ,tap'pi g 24at the inductor 17, the anode 'o f the t'ube being cenkcted thr'odh theintermediate-frequency circuit 4 to the live (i. e. unearthed) end ofthe circuit tuned to the local frequency. As an alternative, though thisis less advantageous, the screen-grids may be connected to the upper endof the said circuit and the anode alone to a tapping of the coil.

th he carra emeae h e q i e .r e i taqmry. co

sion conductance is obtained and the radiation of the 'os'cilla'tion'sfrom the aerial, atlef ast the medium-wave region, may be readilyreduced to below the perr nissible limit.

What we claim is:

1. A co'nverterjfor combining anincomingele ctric wave with localoscillations to produce an inter1 nediate-frequency wave, comprising anelectron discha'r'g'e tube havi a c thode, a, ,cg rq usr d atecreeu gridelectrode and an anode electrode, means to generate local oscillationsctricaily coniii 7.- :1 |.v enoof sai tor. respectively, 1: 1m.- i l, '2t i 1m 1. 13" eonnected ewe a1 cathode and said secp m I said inductorand capaci'tan' rov dingfa resonant circuit 6 i 'oi ejt q i y H 4 c l'sq'i l' ion' am l" conne "fir t istmm l We said "electrodes to g 'afirst one of ai 1e11r6de to I said ei'eerricany connecting anothenof eof said, termihals, fa ta'p'ped, 'feed- 1 a ing the ends thereofconnected 'ele'ctriclly to "s'aidcathode an i c n r the b ,P QY i feedhack sust HS, ine'ans cannep d m g ng said ,etj'w Hem (5n said feedbaclccoil and m ar de, and means cougledjt'ofs'aid anon c't de to derivesaid intermediate-frequency Wayether from 1. a A A verte'r as 'r'ned inclaim '1, in which said first electrode issaid s'creengrid'. I h v 3., Aco ter as laij'edjn lai 2,, in, which said anode is connected for highfrequencies to said first terfl v I 4' 'A c'onvc'riter as cl'ai in whichs'aid .fq.. 9 1 v.. d rm 7,. j 'qi encx-fl transformer hav ng primarywinding connected a I. it r k. t a

ctly in series wlth said anode.

References Cited in the file of this patent

